Topographical modification of the marine atmospheric boundary layer by a bend in the west coast

A summertime maximum of large horizontal scale in marine boundary layer wind speed is centered on Cape Mendocino, where the California coastline bends inward 28 degrees. Satellite-derived and aircraft observations of this maximum during June 1994 when the Monterey Area Ship Tracks experiment was conducted and June 1996 when the Coastal Waves 96 experiment took place are presented. On average, the high-speed region extended over 300 km offshore, exceeding the 100-km average Rossby radius, and 1000 km alongshore. While the maximum was present during much of the time period considered, its extent varied. Crossshore, the region extended from 200 to over 1000 km from Cape Mendocino, or 1.5 to 5 times the Rossby radius. Alongshore, it extended from 600 to over 2000 km. Instances when the maximum was present were associated with downcoast winds north of the Cape. To explore whether the high-speed region is a response to the large coastline bend at Cape Mendocino, the MABL was modeled as a shallow, transcritical flow along a bent coastal wall. Model results were compared to data from one flight of the Monterey Area Ship Tracks and Coastal Waves 96 experiments supplemented by satellite observations. For these two cases, the observed high-speed, shallow supercritical region of significant crossshore extent was reproduced by the model. The model was then used to explore the effects of varying the 1) coastal bend angle to study the onset of a high-speed/supercritical region and 2) initial Froude number to see if this might cause the observed variation in the size of the maximum. Even flows of low initial Froude number developed a supercritical region whose crossshore and alongshore extent increased with bend angle, though not indefinitely. As the initial Froude number increased, the high-speed region extended past the Rossby radius. The model high-speed region was often displaced offshore in the south, a feature of the June 1996 average, because friction more effectively slowed the thin nearshore flow. As observed in some of the satellite maps, the model high-speed region could extend north of the Cape. In addition to presenting new observations of a high-speed region off the West Coast in summertime, this study suggests that it is the response of a shallow-water flow to the large bend in the California coastline.